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W. D. COOLIDGE. INCANDESCENT CATHODE DEVICE. APPLICATION man 050.4,1917.

1 326,029. Pafented Dec. 28, 1919.

Inventor: William D. Coolidge,

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WILLIAM D. COOLIDG E, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERALELECTRIC COMPANY, A CORPORATION OF NEW YORK.

INCANDESCENT CATHODE DEVICE.

Specification of Letters Patent.

Patented Dec. 23, 1919.

Application filed December 4, 1917. Serial No. 205,439.

To all whom it may concern Be it known that I, lVILLIAM D. CooLmon, acitizen of the United States, residing at Schenectady, in the county ofSchenectady, State of New York, haveinvented certain new and usefulImprovements in Incandescent Cathode Devices, of which the following isa specification.

The present invention relates to thermionic devices, and particularly toX-ray tubes operating by an electron discharge generated substantiallyindependently of gas ionization.

X-ray tubes operating without appreciable positive gas-ionization haveheretofore been provided with targets, or anodes, of re-- fractorymetal, such, for example, as tungsten. An X-ray tube thus provided iscapable of operation with a source of alternating current fora shorttime, or more strictly speaking with an amount of energy input whichwill not heat the focal spot upon the anode receiving the cathode dis-.ligible electron emissivity, even when the charge to a temperaturematerially in excess of about'750 C., at which temperature the electronemission from heated metals begins to be appreciable. As soon as anypart of the anode at the focal spot is heated to a sufiiciently hightemperature to emit electrons copiously, a so-called inverse electronstream is emitted from the heated part of the anode and strikes theopposite part of the tube, usually the glass wall of the bulb.

The result is local heating, cracking of the glass and destruction ofthe tube.

I have discovered that an anode consisting of a refractory metal, suchas tungsten, backed by copper, may be freed sufficiently from gas to beused in a tube operating with a substantially pure electron dischargeand that such an anode has a substantially negtungsten at the focal spotis heated to bright incandescence or in fact up to its melting point. Myinvention includes both a new process of preparing an electron dischargetube and a new tube which will continue to rectify its own current evenwhen the anode becomes in part highly heated.

In the accompanying drawings Figure 1 is a somewhat diagrammaticperspective view of the X-ray tube as a whole; Fig. 2

is an enlarged sectional view of the electrodes.

The tube as shown in Fig. 1 comprlses a glass bulb 1 having oppositelyextending arms 2 and 3 into which the electrodes are sealed. The cathodeas shown more clearly in Fig. 2 comprises a spiral filament -l oftungsten, or other suitable refractory metal, connected to leading-inconductors 5 and (5, which are sealed into a glass stem 7 and joinedrespectively to external contacts 8 and. 9. The cathode spiral may beheated by any suitable source of low voltage current, as for example abattery 10, in circuit with a switch 11.

Surrounding the cathode 4: is a focusin device 23 comprising acylindrical portion and a hemispherical portion. The object of thisdevice is to cause the cathode rays to converge, the focus of the pencilof rays being directed upon a target or anode 12 located opposite thecathode. The focusing device 23 is supported upon the wire 13 andtogether with one of the cathode conductor-s 5 is attached to a shortiron tube 14 carried by the stem 7. The conductor 6 passes through ahole in the end of the tube 14. The three Wires, 5, 6 and 18 are held inplace by a bridge 15 suitably insulated with mica, or the like. V

The anode comprises a tungsten button 15 and a mass of copper 17molecularly joined to the tungsten. This copper mass is treated toremdve dissolved gases, preferably by adding to the copper while moltena small quantity of oxidizable boron material, as for example, boronsub-oxid or boron carbid. The degasified copper is then melted in avacuum in contact with the tungsten body and under these conditionsthoroughly wets the surface of the tungsten and forms a perfectmolecular union therewith. This form of anode is described in my priorPatent #1.162,339.

Theanode stem may be mounted upon a split iron tube 18 by iron wire 19.This tube 18 is in turn inserted into a glass tube 20. The anode stem 17is joined to a short platinum ring which is sealed into the glass wallof the arm 3 of the tube. The body of copper carrying the button oftungsten projects outside of the tube. External to the X-ray tube theanode stem is in effective thermal relation with a radiator21..comprising a series of plates spaced apart, consisting of copper, orother good heat conducting metal.

The object in making of considerable length the anode stem 17,, which iscontinucathode.

ous with the copper backing member 17, is to provide without appreciablesacrifice of heat conductivity an extended narrow space within theevacuated bulb between the stem 17 and the wall of the anode arm 3 tosuppress the passage of electrons to the glass wall adjacent the anodeseal. The narrowness of this space insures a high space charge for straysecondary cathode rays, that is, the impeding effect of the electricalcharge of the electrons upon each other is relative high, therebypreventing the accumulation of high negative charges upon the glass walladjacent the anode seal whichwould result in rupture of the glass athighimpressed voltages.

The tube thus assembled exhausted to a good vacuum and then dry hydrogengas at a pressure of about an atmosphere is intr( duced, and is againexhausted. The tube is then again filled with hydrogen at aboutatmospheric pressure and heated to about 350 C. for about fifteenminutes. The tube thereupon is exhausted to the lowest pressureobtainable with a molecular pump, or con.- densation mercury vacuumpump, the conduit connecting the bulb to the pump having a liquid airtrap. In my opinion the beneficial effect of this hydrogen treatment isdue to theremoval of oxygen from the target.

VV hen the exhaust has been made ascomplete as possible and the tube hascooled, a discharge is cautiously started, While the cathodeis heated toincandescence, thereby disengaging further amounts of gas from theanode, as well as incidentally from the This treatment is continued,.thegas evolved being continuously removed, as described in my prior Patent1,203,495 of October 31, 1916. When finally a discharge may beobtained-between cathode and anode, substantially free from accompanyingevidences of gas ionization, the tube is sealed off from the pump.

I have found that a tube thus prepared of current, such as the secondaryof a transformer 22 and that the tube with the cathode heated toincandescence will conduct but such half waves of current as arenegative with respect to the cathode and will not conduct waves ofopposite polarity although the tungsten button upon which the cathoderays are focused is heated to bright incandescence. In fact, in somecases I have actually melted tungsten at the focal spot during theoperation of the tube without obtaining an inverse discharge at theanode with its accompanying manifestation of bombardment of the wallnear the cathode end of the tube by cathode rays.

In my opinion this surprising phenomenon canbe explained by the factthat in spite of the great care exercised to remove dissolved gases fromthe copper, some gas, particularly a trace of oxygen, remains dissolvedin the copper, orcombined therewith, and that some of this oxygen actsupon the heated tungsten and produces some changes, chemical orphysical, at the incandescent tungsten surface, suppressing or greatlyinhibiting electron emission. Some of this oxygen may be evolved in thegaseous state from the incandescent tungsten, but is immediatelyabsorbed again by the cooler copper in the vicinity and may then againdiffuse through the copper to the tungsten button.

I have found that the bulb of a tube embodying my invention may be madevery much smaller than for the former types of X-ray tubes. Becauseofits simplicity and ease of operation, an X-ray tube embodying my presentinvention is well adapted for portable X-ray outfits, particularly forfield work in military operations. It is also well suited for bedsidework in hospitals.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is

1. An electron discharge device comprising an envelop, a cathode, meansfor independently heating said cathode to incandescence, and an anode,the electron emissivity ing an envelop, a cathode, means forindependently heating said cathode to incandescence, and an anode,consisting of tungsten with a backing of copper, the electron emissivityof said anode being negligible even when said tungsten is heated toincandescence.

3. An X-ray tube comprising an envelop, a cathode, of refractorymaterial, means for heating said cathode by passage of current, meansfor focusing an electron discharge from said cathode, an anode Ontargetfor receiving said discharge, the electron emissivity of the fohal areaof said target being negligible even when heated to incandescence.

4. The combination of a source of alter- -ray tube which consists infilling said tube with hydrogen -gas, heating the bulb to a temperatureof several hundred degrees centigrade, evacuating said hydrogen fillingto a pressure so low that the effects of gas ionization will beinappreciable when a discharge is passed through said tube, thensubjecting the tube to passage of current while continuing saidevacuation and finally sealing said tube when the discharge issubstantially unaccompanied by gas ionization.

6. An X-ray device comprising an inclosing evacuated envelop, a cathodeoperable at incandescence, an anode comprising a charge-receiving memberof refractory metal and a backing member of copper, the electronemissivity of said anode being negligible at incandescence of saidcharge-receiving member, and a heat-conductive stem extending from saidanode through the Wall of said envelop, said envelop being shaped toprovide an extended narrow space about said anode stem.

7 An X-ray' device comprising a cathode operable at incandescence, ananode comprising a button of tungsten and a backing of coppermolecularly joined to said tungsten, means for directing an electrondischarge from said cathode upon said tungsten target, an anode stemadapted to effectively conduct heat away from said copper backing and aninclosing sealed envelop having an arm shaped to provide an extendednarrow space about said anode stem, said device being freed from gassufliciently to enable an electron discharge to occur therein Withoutappreciable gas ionization.

In Witness whereof I have hereunto set WILLIAM D. COOLIDGE.

